The cells must know how and where to switch genes on and off as differentiation proceeds. How do they know? The cues come from the other cells and tissues around them.
Some of these signals are delivered as chemical messages, which, diffusing through the mass of cells, serve to define a kind of spatial grid that lets cells know where they are in the overall embryo and thus what their fate should be.
Imagine that a cell, or group of cells, at one place in the embryonic mass switches on a gene that produces some protein. And suppose that this protein can diffuse out of the cell, like water leaking out of a paper bag, and into other cells. Then the concentration of the protein throughout the embryo varies gradually from place to place, being greater nearest the cells that produce it and slowly diminishing with distance. If you could measure the protein concentration, you’d have some notion of where you are in the embryo relative to the source cells. You’d be able to sense your position. Think of it in the same way as finding your way to the kitchen of a large house by following the smell: the stronger it is, the closer you are.
These “position-marker” proteins are called morphogens, and cells are able to “sense” their concentrations. Morphogen concentration gradients allow regions of the embryo to become distinct from one another.
To see how this can work, let’s forget the human body for a moment and look at the development of a simpler embryo: that of the fruit fly. This humble creature became the paradigmatic representative of “complex life” in the early twentieth century, when its robustness and ease of breeding made it the ideal subject to study the mechanisms of genetic inheritance – an art of which Thomas Hunt Morgan was the master. There are, of course, substantial differences between humans and fruit flies, extending to their genetic and developmental fine print. In particular, fruit-fly embryos, unlike those of mammals, are not initially clusters of separate cells at all. Once fertilized, the ovoid fly egg starts to replicate chromosome-carrying cell nuclei, but just accumulates these around the edges of the egg. The nuclei only acquire their own cell membrane once the embryo has amassed 6,000 or so. This lack of cell membranes in the early embryo makes it particularly easy for morphogens to diffuse through it.
One simple way that gradients of diffusing molecular morphogens can mark boundaries is to think in terms of concentration contours. A contour denotes a threshold: a point where the concentration exceeds a certain value.
The fruit-fly embryo acquires its initial pattern features from morphogen threshold concentrations. Pretty much the first thing it does is to define which end will become the head and thorax, and which end the abdomen. In other words, the embryo acquires a front–rear axis. That is defined by a morphogen protein called bicoid. At the tip of the “head” (so-called anterior) end, the embryo produces bicoid, and this begins to diffuse down to the rear (posterior) end. The concentration falls smoothly from the anterior to the posterior end. Where it exceeds certain values, the bicoid protein will bind to the DNA within the embryo and activate other genes with vivid names like hunchback, sloppypaired 1 and giant (typically named because of the developmental defects that mutations in the genes can produce). How this switching occurs is complicated, not least because it also seems to depend on a gradient of another protein called caudal that diffuses from the opposite (posterior) end. But the outcome is that the embryo becomes quite sharply segmented into regions where different genes are expressed or not. Thus the uniformity of the embryo is destroyed: an anterior– posterior axis is established, along with the segments that will develop into the fly’s head, thorax and abdomen. It seems that similar gradients cause segmentation of the neural tube of vertebrates: the tissues that will become our brain and spinal column.
Gradients in the concentration of proteins bicoid and caudal from opposite ends of the fruit-fly embryo switch on genes at different positions that cause segmentation of the body plan.
Other diffusing morphogens produce other kinds of gradient, defining different axes of the emerging body. For example, a protein called dorsal is involved in setting up the top-to-bottom (dorsoventral) axis of the fruit-fly embryo that distinguishes the region that will become the back (where the larva will ultimately grow wings) from that which will become the belly. In each case, the gradient thresholds may turn particular genes on and off in a series of elaborations that begins with the crudest determinants of shape – the front/back and top/bottom axes, say – and works its way to the fine details.
The idea that chemical concentration gradients might control the development of embryos was first proposed at the start of the twentieth century by Theodor Boveri. By producing a chemical patterning signal that spreads into the rest of the embryo, one cell can determine the fate of other cells nearby. In 1924 Hans Spemann, together with Hilde Mangold, called such groups of cells “organizers”.10 Mangold transplanted groups of cells in amphibian embryos from one position to another and saw that they could induce the development of “out of place” features.
The British biologists Julian Huxley and Gavin de Beer verified the idea of organizers in the 1930s by manipulating the embryos of birds. They proposed that Spemann’s organizers create “developmental fields” of some kind that influence the course of development. Spemann had imagined this “field” as something like the magnetic or electrical fields of physics, but Huxley, de Beer and their contemporaries in this fledgling field of developmental biology suspected that the agent was a chemical one. The notion that these organizing centres define a sense of position within the emerging body plan through the action of morphogen concentration gradients was crystallized in the late 1960s by biologist Lewis Wolpert.
There’s a crucial part of this story that I’ve skipped over so far. The patterning of the fruit-fly embryo is kicked off by the production of the bicoid protein at the anterior tip of the egg. But what causes that production in the first place? How does the bicoid gene know it is at the anterior end?
The answer is that “mother tells it”. While the unfertilized egg is attached to the follicle of the mother fly, specialized cells called nurse cells deposit material needed to make bicoid – specifically, the RNA molecules that mediate the gene-to-protein conversion – into the anterior tip of the egg, so that developmental patterning is all ready to go when the egg is fertilized. Right from the outset, cells in the embryo are dependent on other cells around it to know what to do. It’s for similar reasons that a fertilized human egg can’t develop fully in isolation, if cultured in a test-tube. Implantation in the uterus wall is needed to give it a “sense of up and down”. Ectopic pregancies (within the fallopian tubes) show that such a signal doesn’t have to come from the uterus, however, and we’ll see later whether there might be other ways to do produce it in vitro.
This is why it is strictly incorrect to say – although it often is said – that all the information needed to grow a human being is in the genome of the fertilized egg, which is in turn supplied by the gametes that combined to make it. You could say that the human embryo also needs positional information supplied by its environment – specifically by the uterus lining. Furthermore, any particular cell in the developing embryo depends on receiving information from the surrounding cells in order to keep embryo growth on track. As the transplantation experiments of Huxley and de Beer showed, if you mess with that information then you screw up development, despite the fact that every cell retains its complete “genetic programme”.
Embryo development is thus not encoded from the outset in the genome, as if in some blueprint or instruction book. It relies on a precise expression of genetic information in time and space, which in turn depends on the proper coordination of many cells (including maternal ones) and is subject to chance events during